Unit Conversion Library in excel

[ch1253]

I am an engineer i have to use & create excel on daily basis. Where unit conversion become great headache for me. How can i code a simple library where i can manage a drop down list which will convert the value of some cell into some other unit that I chose. And i want to use this library in every excel that I want to use. Sorry I am very new to excel coding. Pleas let me know if i am pushing the limit of excel

Regards